This invention relates to a multi-level xerographic system such as high light color systems. More particularly, this invention relates to a xerographic system which utilizes multi-beams to overscan each line to generate multiple levels of xerographic exposures.
A conventional raster output scanner utilizes either a light source, a modulator and a multi-faceted rotating polygon mirror as the scanning element or a light source, which serves as both a light source and a modulator, along with a multi-faceted rotating polygon mirror. In a raster output scanner with a light source and a separate modulator, the light source, which can be a laser source, generates a light beam and sends it to the modulator. The modulator receives pixel information and modulates the pixel information onto the light beam. However, in the raster output scanner without a separate modulator, the light source, which can be a laser diode, both generates and modulates the light beam. Then, the modulated light beam is directed onto a facet of a rotating polygon mirror. The rotating polygon mirror reflects the light beam and also causes the reflected light to revolve about an axis near the center of reflection of the rotating polygon and scan a straight line. This reflected light beam can be utilized to scan a document at the input of an imaging system or can be used to impinge upon a photographic film or a photosensitive medium, such as a xerographic drum at the output of the imaging system.
A tri-level printing system is a system which uses two color inks. A typical tri-level system utilizes a single light beam which will be modulated to have two different pixel informations, one for the first ink and the second for the second ink. The single light beam, modulated by two different trains of pixel informations, will expose the photoreceptor plane at three different exposure levels: one level for color ink, one level for black ink and the third level for no printing. It should be noted that for each pixel the photoreceptor will be exposed by only one of these three levels.
Referring to FIG. 1, there is shown a train 10 of different exposure levels on the photoreceptor corresponding to different pixels of a tri-level system. Usually in a tri-level system, level 12, which is the lowest level and usually is kept at 0 volts (ground level), represents black ink and is called black level. Level 14 represents no printing and is called white level. White is a term used for no printing since when there is no printing the color of the paper which usually is white will be shown. Of course, if a different color paper is used, white level represents the color of the paper. Finally level 16 represents a second ink which can have any color other than black and the color of paper.
The modulation of a single light beam with two different pixel informations can be achieved through various methods such as amplitude modulation or pulse width modulation.
In tri-level systems, the amplitude modulation is based on three levels. Typically in a black and white printing system, the light beam will be modulated to be either On or Off. With amplitude modulation for tri-level, the light beam will be turned On or Off, but when it is turned On, it will have either full intensity for color or it will have a lesser intensity for white. The full intensity creates the highest level 16 of exposure on the photoreceptor, the lesser intensity creates the exposure level 14 and when the light beam is turned Off, it will create the lowest level of exposure 12.
The same result can be achieved by utilizing a pulse width modulation. In Pulse width modulation the width of each pulse determines the amount of exposure. Depending on the width of the pulse for each pixel the photoreceptor will be exposed less or more. For color level 16 the width of the pulse is more than the width of the pulse for the white level 14 and for the black level there is no pulse. Therefore, if the pulse has a shorter pulse width the photoreceptor will be exposed less (white level) and if the pulse has a longer pulse width, the photoreceptor will be exposed for a longer time and therefore it will reach to a higher exposure level 16 (color level).
The problem with amplitude modulation is controlling the color level and the white level. A slight variation in the color level causes the color to become either lighter or darker. However, the problem with variation of the white level is more severe than the variation of the color level. If the white level varies, instead of no printing, a pale color or a pale gray will be printed on the paper. Therefore, keeping the white level at a precise level is more critical. Typically, to control the white level the power of the laser diode will be divided into small steps which will be used to adjust the white level. The more the number of the steps, the more the control over the white level.
Also, for the color level, the power of the laser diode is divided into steps. However, the number of the steps for the color level is less than the number of the steps for the white level. Typically, a single channel laser diode is utilized to produce a light beam for both the white level and the color level. This requires the laser diode to have a high power adjustment (high number of steps) for the white level and also a reasonable power adjustment for the color level which usually is a difficult requirement to be placed on a single channel. Also, since the single channel has to produce the light beam for both levels, it has to work in a power range which covers both levels. This also adds to the complexity of the power adjustment for both levels.
The problem with pulse width modulation is the required high frequency. In pulse width modulation, for every change of level (color change) a pulse should be generated. Therefore, for high resolution printing systems which have higher number of pixels per inch (higher number of color changes), if a pulse width modulation is used, the frequency will be very high.